A microwave dielectric ceramic is a new functional electronic ceramic material that has been developing rapidly in recent three decades, which features low loss, a small frequency temperature coefficient, a high dielectric constant, and so on. This microwave ceramic material may be made into a dielectric resonator, a dielectric filter, a duplexer, a microwave dielectric antenna, a dielectric frequency oscillator, a dielectric waveguide transmission line, and other components. These components are widely applied to various fields such as mobile communications, satellite television broadcast communications, radar, satellite navigation and positioning system.
The microwave dielectric ceramic material applied to a modern communications technology needs to meet the following performance requirements: (1) A serial dielectric constant ∈r to meet requirements of different components for different frequency bands; (2) Minimum dielectric loss tan δ or high quality factor value Q (Q=1/tan δ, often represented by Q×f, where f indicates a working frequency or a resonant frequency, the Q value of a material of practical value should at least be over 1000, and Q×f>5000 GHz. For example, Q×f of an Al2O3 based material is up to a million) to ensure an excellent frequency selective feature; (3) Close to zero temperature coefficient of resonant frequency (TCF) (τf) (for most dielectric materials, the τf depends on a temperature coefficient of a dielectric constant), so as to ensure fine temperature stability of a component; (4) Low-cost environment-friendly material.
In order to meet needs of miniaturization and integration of a component, a focus and an emphasis of current research are to find, prepare and develop new microwave dielectric ceramic that features a high dielectric constant (∈r>45, a high K value), low loss (Q×f>5000 GHz), a close to zero temperature coefficient of resonant frequency (TCF=0 ppm/° C.), a low cost (containing no or a small amount of precious metal), environment-friendliness (at least lead-free, containing no or a relatively small amount of toxic raw materials).